The invention relates to an evaluation circuit for the processing of a received differential signal of an optoelectronic sensor, in particular of a light grid, at least having one differential amplifier for the amplification of the received signal. The invention further relates to a corresponding signal processing method.
The use of light grids is known in automation engineering and safety engineering in which a plurality of parallel light rays are periodically transmitted and monitored to carry out an areal monitoring. The received signal of each light ray is ultimately compared with a threshold value within a simple comparator stage in order to trigger an object detection signal or a switch-off signal when the threshold is not reached or is exceeded. The signal processing chain on the reception side typically contains a photodiode as the light receiver, a transimpedance amplifier with a band-pass property, a differential amplifier stage and the comparator stage for each reception channel.
With respect to the plurality of the reception channels, the most cost favorable realization of the evaluation electronics is desired. It is therefore desirable to combine the analog received signal processing for each reception channel at least in part in one integrated circuit, with these circuits being connected parallel to an analog bus and with one single common comparator stage being provided for all reception channels of this analog bus for the further reduction of the manufacturing effort.
These integrated circuits can, however, have comparatively large tolerances as a result of production spreads so that the differential output stage can have an unwanted difference in the output current signals with a short-circuited input. This offset can, in particular with a further amplification, result in the predetermined threshold being accidentally exceeded at the associated light receiver even without any incident light and thus, for example, no switch-off signal being triggered despite the interruption of the light ray in question.
This danger could admittedly be eliminated in that a high pass is interposed before the comparator stage. However, it has been found to be difficult to find a cut-off frequency for this high pass which satisfies the demands of the total system. Too low a cut-off frequency can result in the variation of the respective offset likewise being transmitted up to the comparator stage and a switching of the comparator being effected there on a switch-over between the different reception channels. A high cut-off frequency of the high pass, on the other hand, necessarily lies in the transmission range of the band-pass of the reception channel or of the integrated circuit. Its band-pass limits are also necessarily subjected to large fluctuations. The interaction with the interposed high pass can therefore result in a band-pass of a higher order with a non-defined transmission function so that the spikes resulting therefrom in the time range can in turn trigger an unintentional switching of the comparator stage.